In such switching converters, the control circuit receives a rectified output voltage via a series regulator via the auxiliary winding, which operates in the conduction phase of the converter. To also guarantee the start-up of the control circuit when it is connected to the output voltage, the supply voltage input of the control circuit additionally lies on the input voltage via a current path of a relatively high ohmic resistance.
The basic problem of the present invention arises when a broad range of variation of the input voltage is present, at which the converter is to operate. The input voltage is usually obtained by rectifying a power supply and is also called intermediate circuit voltage. If the alternating voltage is in a range between 85 V and 264 V, a direct input voltage of approx. 115 V to 373 V is obtained. A certain minimum voltage of the auxiliary power supply, for example, 15 V, is required for the control circuit. This voltage must be guaranteed at the lowest power supply. However, it follows from this that a high direct-axis voltage component of about 34 V is present on the series regulator at the highest input voltage occurring, so that a power loss of 0.84 W is generated in case of a control circuit power supply of, for example, 25 mA. Such a power loss is undesirable especially in case of small devices.
An embodiment according to the state of the art is explained in greater detail in FIG. 1. An alternating input voltage UE lies via a controlled switch S on a primary winding WP of a transformer UET. The switch S is controlled by a control circuit AST, this control circuit receiving information on the current through the switch S and the primary winding WP by means of a sensor resistor Rsh.
On the secondary side, the voltage of a secondary winding WS is rectified by means of a rectifier diode D1 and smoothed by a capacitor Ca and is available as an output voltage UA. A voltage sensor SPS sends a regulating signal SR to the control circuit via an optocoupler in order to bring about voltage regulation in the known manner. However, regulation may also take place from the primary side, and input/output currents can also be included in the regulation. The control circuit receives its operating voltage in two different ways: On the one hand, the direct input voltage UE is sent via a protective resistor Rs to the control circuit AST, using a storage capacitor Cs is this case. On the other hand, the forward voltage of an auxiliary winding W1 is rectified by means of a diode D2 and a capacitor C and is sent as a voltage Uhz to a series regulator LAE, from where the regulated voltage likewise reaches the power supply input of the control circuit AST via an uncoupling diode D3. The primary mass is designated by MP and the secondary mass by MS.
If the switching converter according to FIG. 1 is connected to the power supply, it first receives the voltage present on the storage capacitor Cs for start-up and after the run-up of the combinational switching circuit, this will supply itself with the auxiliary supply voltage via the auxiliary winding W1, the diode D2 and the series regulator LAE. As was mentioned above, a high direct-axis voltage component also lies on the series regulator LAE in case of a high input voltage, which leads to undesired losses.
The following documents shall also be cited below in connection with the general state of the art:
WO 93/23974 shows a switching converter, which has an auxiliary supply winding. An auxiliary supply voltage can be obtained via this winding in the flyback converter mode. Moreover, an image of the output voltage can also be obtained from this winding for regulating purposes by the use of two diodes. Moreover, the generally common power supply from the input voltage via a voltage divider is likewise present.
DE 3 419 972 A1 shows a combinational switching circuit, which can optionally be operated at a power supply voltage or a battery voltage. Just as in WO 93/23974 mentioned above, an auxiliary winding is provided here as well, which supplies an off-state voltage for supplying the control unit (after rectification). Furthermore, this combinational switching circuit has two separate primary windings, wherein one primary winding is provided for operation with the rectified alternating power supply and the other primary winding for the operation with a 14-V battery. Finally, yet another auxiliary winding may also be present, whose voltage is used for regulating purposes and is supplied to a control circuit for this purpose.
The document JP 09-285 121 discloses two mutually independent converter systems, which do, however, form a functional unit and are to be synchronized with one another such that a defined sequential sequence is obtained during the voltage run-up. Only one of the two converter systems has an auxiliary winding, whose voltage is used for the power supply of the control circuit after rectification. The other converter system cannot start by itself at all but requires the second, run-up converter system.
The switching converter according to U.S. Pat. No. 4,156,273 has two auxiliary windings. One of these auxiliary windings supplies, after rectification, a supply voltage for the control circuit, and this supply voltage can be switched on or off via a controlled switch. The other auxiliary winding has nothing to do with power supply, but the voltage derived from it shall affect the switching behavior in terms of a desired curve shape.
In U.S. Pat. No. 4,630,186, a bipolar transistor, which exhibits a relatively sluggish behavior, is used as the switching transistor. To make possible the fast switching off of this switching transistor, an auxiliary winding is provided, which generates a countervoltage, which is applied to the control electrode of the switching transistor via another switching transistor at a suitable point in time. Yet another auxiliary winding supplies a voltage, which is to affect the switching behavior. Together with a rectifier diode and a capacitor, this auxiliary is called a voltage correction circuit.